Abstract: Based on the conventional surface observation data and the NCEP reanalysis data，a rainstorm process on August 16～18，2015 in the east Sichuan Province was analyzed by using the ageostrophic wet Q vector(Q^*) ．500hPa upper trough，700hPa Southwest Vortex(SWV) and 850hPa low－level jet were the main systems causing this persistent rainstorm．The results show as follows:(1) The convergence area of Q^*divergence which can better diagnose the area of future rain－band，is coincided with the rain－band after 6 hours，SWV and the rainstorm center locate west by south and south of the convergence area．(2) SWV and the rain－band after 6 hours locate in the southeast and west of the positive vorticity area of Q^*，but the diagnosibility of Q^*vorticity is not as good as Q^*divergence．(3) The large－ scale forcing is the predominant factor during the developing stage of the SWV，however，the meso－and micor－scale forcing is dominant during the intensifying stage of the SWV．(4) The vertical distribution of Q^*divergence can reflects SWV's vertical distribution that SWV locates in the southwest of it．